1. Field of the Invention
The present invention relates to a piezoelectric composite material and more particularly to a piezoelectric material obtained by compounding a ferroelectric ceramic powder and a polymeric binder.
2. Description of the Prior Art
General attempts to obtain piezoelectric materials by compounding polymers and ferroelectric ceramic powders have been known [Mat. Res. Bull., Vol. 13, pp. 599-607, (1978), Pergamon Press Inc. (U.S.A.)]. However, ferroelectric ceramic powders prepared by conventional processes such as those described below have been attended by conspicuous difficulties. That is to say, ferroelectric ceramic powders prepared by grinding ferroelectric ceramics prepared by a solid phase reaction under applied heat such as titanium-containing ceramics (e.g., BaTiO.sub.3, PbTiO.sub.3, PbZrO.sub.3 -PbTiO.sub.3 solid solutions, etc.) or single crystals such as potassium sodium niobate (PSN) and the like, using a ball mill, an oscillation mill or the like, and adjusting the resulting powders so as to obtain a desired size distribution barely provide even low piezoelectric properties in a composite material which is quite contrary to what one would expect based on the piezoelectric characteristics of the ferroelectric ceramics themselves. In addition, this type of composite material is fragile and hard to mold because it lacks pliability, and moldings thereof are heavy and expensive. As a result of these drawbacks the point at which the use of composite materials of this type is practical has, for all intents and purposes, not yet been reached.
Extensive research directed to determining the origin of the deterioration in properties as described above has lead to the conclusion that in the crushing process (pulverization) carried out after the solid phase reaction or the preparation of single crystals, structural fractures arise in the microcrystals resulting in the formation of numerous domains and subsequently to the formation of distorted phases. It is almost impossible to force all the resulting domains and distorted phases to line up in the same direction as that of the applied electric field even when a voltage close to the maximum voltage which the composite material can resist without dielectric breakdown is applied to polarize the composite material. This is because the electric field capable of acting effectively upon the individual ceramic grains compounded with the polymer is markedly reduced to several tenths or several hundredths of the electric field in the compounding state, taking into account the ratio of the dielectric constant of the polymer to that of the ceramic. Therefore, admixing of ceramic powders or single crystals with polymers cannot impart piezoelectric properties to the resulting composite materials to an appreciable extent.